1. Field of the Invention
The present invention relates to usage of MF and MIMO in cellular communication systems, e.g. HSPA networks.
2. Related Background Art
The following meanings for the abbreviations used in this specification apply:    CQI channel quality indicator    HO handover    HSDPA high speed downlink packet access    HSDPA high speed downlink packet access    HSPA high speed packet access    LTE long term evolution    MF Multiflow    MIMO multiple input multiple output    PCI precoding control indicator    RACH random access channel    RAT radio access technology    RNC radio network controller    RRC radio resource control    SF-DC MF single frequency dual cell multi flow    SINR signal to interference and noise ratio    TTI time transfer interval    TX transmit    UE user equipment    UL uplink
Both MIMO, i.e. beamforming or adaptive single-/multi-stream MIMO, and MF, i.e. simultaneous transmission from two cells to one terminal, are promising features in HSDPA. However, they may in some cases not be used simultaneously. More precisely, if a terminal has NR receive antennas, it may only receive up to NR data streams instantaneously, originating from one or multiple cells. For example, if a terminal has 2 receive antennas, it may receive dual-stream MIMO from one cell, or a single stream from two cells, but not MIMO and MF simultaneously from both cells.
According to the example shown in FIG. 1, a UE has NR=4 receive antennas, so it may receive up to 4 data streams per carrier. A base station BS1 of a cellular communications network has NA=4 transmit antennas and may transmit up to 4 data streams per carrier to each terminal. Similarly, a base station BS2 of the cellular communications network has NA=4 transmit antennas and may transmit up to 4 data streams per carrier. The BS1 and the BS2 are controlled by an RNC of the cellular communications network.
Hence, a problem arises that when using MIMO and MF it has to be ensured that the overall number of transmitted data streams on one carrier meets UE capabilities.
Currently, fixed signal margins are used to determine whether a terminal (UE) is in an HO area and hence applicable to MF or not.